the brink this morning. The israelis have been firing hundreds more rockets into gaza, with hamas firingback. And a decision about a ground invasion could come this weekend. We have team coverage this morning. Christiane amanpour is in jerusalem. We're going to start with alex ma marquardt, in gaza. Reporter: We have been watching the warplane as they continue to pound this enclave. Some of hamas' most important buildings have been targeted. And many are wondering whether this aerial campaign will soon become a ground invasion. This is life in gaza these days. A relentless stream of booms and whooshes, as israeli missiles land and militant rockets are launched. In israe crosshairs overnight, hamas' seat of power, blown up. Another hamas building targeted from the sea. All reduced to rubble. In israel on friday, sirensn jerusalem, as the sacred city was targeted for the first time in more than 40 years. A rocket fell ten miles short. No one was hurt. Two days in a row, residents of tel aviv, the commercial center, ran for cover, as sirens sounded. You know, life is continuing. We're not going to let hamas or other people in gaza to interrupt my way of life. Reporter: Three israelis and almost 40 palestinians have been killed in the past 4 days. Around half of them, civilians. We saw the wounded rushed to gaza's biggest hospital. Its intensive care unit filled with what doctors say are just civilians. Are you able to cope if this escalation continues? If it continues, we can't. Of coue, we can't. Reporter: It looks set to continue, with columns of israeli tanks and troops massing along the border with gaza, fueling fears here of a ground war. The israeli army has called up 20,000 reservists. It's clear that israel is preparing for a drawn-out nflict. Local authorities have been told to be ready for seven weeks of fighting. Dan? Seven weeks. Alex, thank you. Les go to jerusalem. Coming under fire for the first time. Abc's global affairs anchor, christian amanpour is there. I've been to israel during times of war. And often the mood is less panic and more grim determination. How would you describe what you're hearing and seeing from people there this morning? Reporter: Well, certainly, what we're seeing, obviously, is so anxiety. But I think grim determination, as well. Not just here in jerusalem but in tel aviv, as well. We were in the old city of jerusalem. And it's is bustling there. Nobody there seems to be too afraid. There's a lot of tourists out there. A lot of local people out. That seems to be going on and proceeding as normal. What's happening is the equation is somewhat changed. Now, when the israelis hit gaza, the hamas has decided that they're going to hit their long-range missile. And for the first time this week, they have targeted jerusalem and tel aviv. Fortunately, no casualties. But this is a new element for the tit for tat that's going on between the two sides. "The new york times" reporting this morning that the white house is worried about an israeli ground invasion, perhaps playing into the hands of hamas, the militants that run the government there. Are the israelis worried about that? Reporter: I think, if you talk to the israelis, they say we're going to do what we have to do. It's our right. We must. We have to defend against the rockets falling in and injuring our people. But I think underlying, there is obviously a concern about if they do go in on the ground, how will that end? And what will that mean? And what will be the backfire, the blowback, on israel? The united states is concerned. It has relations with israel, but also with the other arab nations in this part of the world. This is the first such israeli/palestinian conflict since the arab spring. And the arab leaders are standing shoulder-to-shoulder with hamas, than they did ever before in these situations. If there is a ground invasion, first and foremost on everybody's mind, is what if there are civilian casualties. Unacceptable numbers of civilian casualties. That has potential to destabilize this part of the world. Destabilize this part of the world with its only two friends here, egypt and jordan. The new middle east. Christiane amanpour, we're so happy to have you there reporting. Thank you.

This transcript has been automatically generated and may not be 100% accurate.

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